Incomplete Freund's Adjuvant (IFA) is an emulsion-based adjuvant that has been widely used. It is a water-in-oil emulsion consisting of a light paraffinic mineral oil of low viscosity that is stabilized by Arlacel.TM. mannide monooleate as a stabilizing emulsifier. IFA has been administered as an adjuvant to more than a million persons worldwide as a constituent in influenza, poliomyelitis, cholera, typhoid and tetanus toxoid vaccines. Although this adjuvant appears very safe, a small number of those administered vaccines containing IFA showed toxic reactions, and hence efforts have been made to find suitable substitutes.
Other emulsion-based adjuvants have thus been developed. Among these is the Syntex Adjuvant Formulation (SAF) which utilizes a detergent as an emulsifier.
Besides their ability to act as adjuvants, emulsions are also important in formulating cosmetics, and emulsions are often found among food products, such as mayonnaise; however, the present invention is directed to emulsions useful to deliver immunogens or antigens, as well as therapeutic substances.
Emulsions are stabilized suspensions either of oil in water (o/w, i.e., water is a continuous phase with oil as a discontinuous phase) or water in oil (w/o, wherein water is a discontinuous phase and oil is a continuous phase). In both, the oil and water phases are prevented from separating by addition of an emulsifier. Emulsifiers are "amphiphiles" or amphipathic molecules containing a hydrophobic region and a hydrophilic region. Typical examples are detergents and phospholipids, such as lecithins.
It is not unknown to include vesicles such as liposomes in or as emulsions for various purposes. For example, such preparations are described in U.S. Pat. No. 3,957,971; UK patent GB 2,079,179; and U.S. Pat. No. 5,256,422. It appears that the '971 and '179 patents relate to liposomal compositions per se; thus, the emulsions are those of liposomes themselves in an aqueous suspension. This '422 patent relates to w/o emulsions. In all cases, an emulsifier is added to the preparation in addition to the liposomes. Yoshioka, T. el al., J Drug Targeting(1995) 2:533-539, similarly, disclose a water-in-oil emulsion containing nonionic surfactant vesicles (niosomes) which employs an additional emulsifying agent. In the Yoshioka paper, the same molecule was used to provide the amphiphilic component of the vesicles as was also used for the emulsifier.
Vesicles have been used alone to stabilize cosmetic emulsions. U.S. Pat. Nos. 5,489,426 and 5,439,672 describe oil-in-water emulsions stabilized by niosomes--i.e., vesicles similar to liposomes formed from nonionic amphiphiles (further described below), although ionic amphiphiles may be included as well. The oil phase in these compositions is a fatty acid ester. The niosomes are said not only to stabilize the emulsion, but to exhibit stability themselves in the context of the emulsion. U.S. Pat. No. 5,055,228 describes oil-in-water emulsions for a variety of uses that are stabilized by the presence of vesicles, which vesicles may contain either ionic or nonionic amphiphiles or both. A major advantage of this composition is said to be the enhanced stability of the vesicles due to the presence of oil droplets. After formation of an illustrative emulsion (column 6) 36% of the glucose entrapped in the vesicles was released after 5 days, while a similar dispersion lacking oil droplets released 45% of the glucose contained in the vesicle after 5 days.
Liposomes per se have also been used as carriers in vaccines. See, for example, PCT application WO 93/10763. In one particular application, the adjuvant is an amphipathic lipid which becomes incorporated into the liposomes per se. See Wassef, N. M. et al., Immunomethods (1994) 4:217-222.
There remains a need to add to the repertoire of emulsions that can be used adjuvants, vaccines, and the like without negative reactions on the part of subjects administered them and with maximal ability to stimulate a desired therapeutic or immune response. The present invention provides such compositions.